Fixed zero exit code when plugin not found

Github user chinedufn reported the following problem:

I started adding the mdbook-linkcheck plugin to my book build process and was confused when CI was still passing even though I had forgotten to install the plugin in my ci config.

#!/bin/bash -eo pipefail
(cd book && mdbook build) && cargo doc --no-deps --document-private-items -p foo && cp -R target/doc book/book/api
2019-03-25 16:25:00 [INFO] (mdbook::book): Book building has started
2019-03-25 16:25:00 [INFO] (mdbook::book): Running the html backend
2019-03-25 16:25:01 [INFO] (mdbook::book): Running the linkcheck backend
2019-03-25 16:25:01 [INFO] (mdbook::renderer): Invoking the "linkcheck" renderer
2019-03-25 16:25:01 [WARN] (mdbook::renderer): The command wasn't found, is the "linkcheck" backend installed?
2019-03-25 16:25:01 [WARN] (mdbook::renderer): 	Command: mdbook-linkcheck
 Documenting tw v0.1.11 (/root/foo-barg/app/crates/foo-bar-cli)
    Finished dev [unoptimized + debuginfo] target(s) in 2.16s

He expected that mdbook's build would fail with a non zero exit code if he tried to use a plugin it couldn't find.

https://github.com/rust-lang/mdBook/pull/1122